This invention is a process relating to the fabrication of integrated circuits, and more specifically to the chemical vapor deposition of thin films on the silicon wafers which serve as the substrate of the circuits.
In the fabrication of integrated circuits, several hundred circuits are formed side-by-side on a single silicon wafer which has been cut from a long single crystal of silicon. Thin films of various materials which serve as the active circuit elements are deposited on the silicon wafer at various stages of the fabrication process. After each film is deposited on the wafer, certain portions of the film are selectively removed, usually by photolithography, so that only a predetermined pattern of the film remains.
A conventional method for forming the various thin films on the wafers is by chemical vapor deposition. In this process, the wafers are placed in a "boat" in a spaced-apart mutually parallel arrangement. The boat containing the wafers is inserted into a long quartz tube which is evacuated at the exit end by means of a vacuum pump. The quartz tube is surrounded by a cyclindrical heating element which directs thermal radiation into the interior of the tube to heat the wafers. A reactant gas introduced at one end of the tube is forced to flow toward the exit end of the tube because of the vacuum pump. As the gas passes the wafers, a chemical reaction occurs and a product of that reaction is deposited as a film on the wafers. In many instances more than one reactant gas is introduced so that a product of a chemical reaction between the two gases is deposited as a film on the wafers.
This conventional process for the chemical vapor deposition of films on silicon wafers has several inherent disadvantages. Because the silicon wafers occupy only a relatively small part of the evacuated tube, considerable reactant gas must be introduced, only a portion of which is necessary to form the thin film.
Since the gas is passing through the tube in a direction perpendicular to the wafer surfaces on which the film is to be deposited, the gas flow is interrupted and unpredictable flow characteristics are generated within the tube. It is only because of a changing flow direction caused by the silicon wafers that gas is forced to travel between adjacent wafers and thus into contact with the wafer surfaces. This unpredictable flow pattern across the wafer surfaces often results in a lack of uniform film thickness on each wafer.
The greatest localized volumetric rate of flow of gas and thus the fastest film deposition rate occurs near the first wafer contacted by the flowing gas. In contrast, the last wafer in the wafer arrangement experiences a substantially reduced rate of film deposition. In order to compensate for variations in the rate of film deposition among wafers in the boat, an inert carrier gas, such as nitrogen, is used to "carry" the reactant gas toward the exit end of the tube in order to enhance the deposition rate on the end wafers in the wafer arrangement. Additionally, since film deposition rate is a function of wafer temperature, an increasing temperature profile is created along the length of the wafer arrangement by varying the voltage to various elements of the heating elements surrounding the tube. By maintaining each wafer at a different temperature, with the temperature of the wafers increasing in the same direction of gas flow, the effect of the localized variation in volumetric gas flow rate on film deposition rate is compensated, and the rate of film deposition and thus the thickness of the deposited film is maintained relatively uniform among the wafers in the boat.
One attempt has been made to improve the conventional chemical vapor deposition process by enclosing the wafer boat in a cylindrical container having a plurality of holes on its outer surface. The end of the container facing the inlet for the reactant gas has a cover placed over it and the other end is open. As the gas enters from the inlet end of the tube, it changes direction and enters the holes in the cylindrical container where it passes between the silicon wafers. The gas is removed from the tube by passing through the open end of the container and out the exit end of the tube in a conventional manner. This process does not completely solve the problem of localized flow rate variations from one end of the wafer arrangement to the other end, and thus both a carrier gas and a temperature profile are required to create uniformity in film thickness within each wafer and from wafer to wafer.